Merge Facebook commit 926a077b14b73c14094de7fc7aa913241b801b4d
authored by Inaam Rana from https://github.com/facebook/mysql-5.6.

This is fix for upstream bugs
http://bugs.mysql.com/bug.php?id=71988
http://bugs.mysql.com/bug.php?id=70500

page_cleaner should work whether or not there is server activity.
Its iterations become a noop when there is no work to do but we
should not tie it to the server activity.

The page_cleaner thread does spurious background flushing
because of conditional sleep between iterations. The solution
is not to make sleep dependent on server activity etc.